Beavan of the New Zealand geological agency GNS Science determined that the tsunami was actually generated by two powerful quakes - the magnitude-8. 1 "outer rise" quake and a magnitude-8 "megathrust" jolt caused by the diving of one plate under another.

Usually the opposite occurs: big "megathrust" quakes on the subduction zone boundary between two plates trigger other quakes on the oceanic plate that is diving or "subducting" under the continental plate.